1. Field of the Invention
The present invention generally relates to a method and apparatus for performing a registration adjustment for correcting picture distortion and color shifts occurring in a projection television and, more particularly, to an interpolation method for improving the registration adjustment process.
2. Description of the Background
Recently, various types of projection television sets for displaying a picture on a large size screen with high precision have been practically utilized.
FIGS. 5A and 5B schematically show a structure of this sort of projection television 1, in which SC indicates a screen on which a picture or image is projected. Individual light projecting units that project each of a red component (R), a green component (G), and a blue component (B) of the primary color picture are provided on the rear side of this screen SC, which constitutes a so-called rear projection television combined with a screen.
The light projecting unit includes cathode ray tubes (CRTs) for producing the respective primary picture light, deflection yokes DY for deflecting electron beams of these CRTs, and focusing lenses CD for focusing the primary color picture light formed by the CRTs onto the screen SC. The respective primary color picture light irradiated from the light projecting unit is projected via a rear-mounted reflection mirror to the screen SC. The screen can be a lenticular screen constructed as a Fresnel lens, for example. It should be noted that when the reflection mirror M is not employed, such as in the structure shown in FIG. 6, the respective primary color picture light is directly projected from the rear side of the screen SC.
As is apparent from the structures shown in FIGS. 5A, 5B, 6, an optical system is so constructed in the rear projection television 1 with such a structure that the picture light is obliquely projected to the screen SC. In other words, since the optical system is asymmetrically arranged with respect to the screen in the rear projection television 1, the picture light projected from the CRTs must be previously distorted in such a manner that the distortion components caused by this asymmetrical arrangement are canceled in order to project proper pictures without distortion.
Distortion of the projected picture image for canceling asymmetry in the optical system will be referred to as registration correction. A circuit for performing this registration correction is employed in the rear projection television 1 having the above-described structure, and the structure of the rear projection television 1 will now be explained with reference to FIG. 7 in which a video signal processing circuit 2 separates a video signal inputted from a video signal input terminal Vin into R, G, B signals. Video output circuits 3-1, 3-2, 3-3 supply the red component (R), green component (G), blue component (B) of the respective primary color pictures to the red CRT, green CRT, and blue CRT, respectively.
A sync separating circuit 4 extracts a horizontal synchronization signal H and a vertical synchronization signal V from the video signal and outputs these sync signals. A horizontal oscillating circuit 5 produces a horizontal oscillating signal in response to the horizontal sync signal H from the sync separating circuit 4 and a vertical oscillating circuit 6 produces a vertical oscillating signal in response to the vertical sync signal V from the sync separating circuit 4. A horizontal deflection output circuit 7 produces a horizontal deflection signal in accordance with the horizontal oscillating signal and supplies the horizontal deflection signal to the horizontal deflection yoke of the respective CRTs. A vertical deflection output circuit 8 produces a vertical deflection signal in accordance with the vertical oscillating signal and supplies the vertical deflection signal to the vertical deflection yokes of the respective CRTs. A high-voltage output circuit 9 produces a high voltage based upon the horizontal oscillation signal and applies this high voltage to the anode electrodes of the respective CRTs.
A registration correction waveform generating circuit 10 generates a registration correction waveform in accordance with the horizontal deflection signal and the vertical deflection signal, by which asymmetry of the projection optical system is eliminated. This correction waveform corresponds to a signal waveform for defining picture or image distortion. A registration output circuit 11 supplies the correction waveform to a sub-deflection yoke DY1 of each of the three CRTs. The sub-deflection yoke DY1 distorts the picture light projected onto the screen SC in response to the correction waveform, so as to cancel the asymmetry of the optical system, thereby forming a proper image without any distortion on the screen SC.
Therefore, one approach to improving image registration in a projection television is to generate a correction waveform according to some non-linear function that can alter the deflection of the beam in the cathode ray tube, so that such altered beam when combined with the image distortion caused by the arrangement of the respective tubes relative to the screen will result in a corrected image. This approach is seen as an analog approach to providing image registration correction.
On the other hand, digital deflection control systems are known, such as those described in U.S. Pat. Nos. 4,672,275 or 4,754,204 or 5,138,442, for example. Thus, it is also possible to provide a digital correction signal for use in such a digital deflection system.
In both of these two approaches there is the problem of the overall size of the circuitry needed to calculate and interpolate the correction waveform, either analog or digital. On the one hand, when providing digital correction demands are placed on the central processing unit of the microcomputer and, on the other hand, the size and speed of the digital memory can become prohibitive.
Interpolation is one method to arrive at the correction waveform and it known that interpolation can become complicated and time consuming, especially higher order interpolation. Similarly, digital calculations of the correction waveform are limited in their accuracy by the bit size of the data word used in the calculations.